The trade is general manager Peter Chiarelli’s most earthshaking move since acquiring the pick that led to Seguin. On Sept. 18, 2009, Chiarelli shipped Phil Kessel to Toronto for a 2010 first-round pick, a 2011 first-rounder, and a 2010 second-round selection. The Bruins drafted Seguin, Dougie Hamilton, and Jared Knight.

Kessel is now the player Seguin hopes to become. Kessel scored 20 goals and had 32 assists in 2013. During the playoffs, the 25-year-old Kessel collected four goals and two assists in the first-round series against the Bruins.

Eriksson is the centerpiece. The 27-year-old Swede is a left-shot right wing. Eriksson finished 2013 alongside Ray Whitney and Jamie Benn on Dallas’s No. 1 line. It was the position formerly occupied by Jaromir Jagr before his trade to Boston.

Eriksson is a smart, speedy, two-way wing. He had 12 goals and 17 assists this past season while averaging 20:07 of ice time, most among Dallas forwards. In 2010-11, Eriksson had 27 goals and 46 assists for a career-high 73 points.

Given Eriksson’s two-way game, he could be a good fit to play with Brad Marchand and Patrice Bergeron. Eriksson can kill penalties and skate on the power play, where he could man the goal line or right half-boards.

“He’s a fast, good, two-way player,” Chiarelli said during a conference call. “There’s a lot of his game that fits into how we play.”

Fraser, 23, is a left-shot forward. Chiarelli said Fraser should challenge for a roster spot. The 22-year-old Smith scored three goals and six assists in 37 games this past season.

Morrow was Pittsburgh’s first-round pick in 2011. The Penguins traded the left-shot defenseman to Dallas for Brenden Morrow March 24. Chiarelli termed Morrow a strong skater and a possible top-four defenseman.

The Bruins never dreamed they’d move Seguin when they drafted him second overall after Taylor Hall in 2010. They believed they had a sniper who’d end up in a Yonge Street building in Toronto with his name on a plaque. During his draft year in Plymouth, Seguin had 48 goals and 58 assists for 106 points.

As an NHL rookie, Seguin had 11 goals and 11 assists in 74 games. He accelerated his development during his second season, when he had 29 goals and 38 assists for a team-leading 67 points.

Seguin’s performance convinced the Bruins’ bosses of his potential. On Sept. 11, 2012, Seguin signed a six-year, $34.5 million extension. The Bruins did not have to give Seguin another deal at the time as he had another season on his entry-level contract.

But Seguin’s play, coupled with extensions given to fellow 2010 picks Hall (seven years, $42 million) and Jeff Skinner (six years, $34.35 million), prompted the Bruins to open up their checkbook. The contract is effective starting in 2013-14.

In hindsight, it was a premature decision. Seguin’s annual average value — he would have carried the second-highest cap hit among forwards after Milan Lucic ($6 million) — was a factor in his ouster. The Bruins have to extend Tuukka Rask and Bergeron, two of their three most important players. Seguin did not fit into the Bruins’ core group.

By trading Seguin and Peverley ($3.25 million) and acquiring Eriksson ($4.25 million annually through 2015-16, according to www.capgeek.com), the Bruins cleared $4.75 million of 2013-14 salary. The Bruins now have approximately $55 million committed to 11 forwards (including Marc Savard), seven defensemen, and zero goalies for 2013-14. The cap is $64.3 million, down from $70.4 million.

Rask could double his previous $3.5 million salary. The Bruins continue to pursue Daniel Alfredsson. Chiarelli spoke with J.P. Barry, Alfredsson’s agent, again on Thursday.

“What you have to understand in this environment right now is the cap goes down $7 million,” Chiarelli said. “You have to make some hard choices and hard decisions. The fact we signed Tyler had nothing to do with us trading him. It was an opportunity to get a very good player, a natural winger, get some good prospects, lower your cap, and maybe to improve in the next market starting tomorrow.”

Seguin’s speed and shot have few rivals in the NHL. But Seguin’s middling hockey sense and battle level were poor fits for his skills. The Bruins repeatedly asked Seguin to increase his competitiveness. He did so only in stretches.

The Bruins also questioned Seguin’s approach to his livelihood. Last Saturday, amid trade rumors, Chiarelli said Seguin had to act more like a professional.

“He came here with much pomp and circumstance,” Chiarelli said. “He played very well for a young player. This year wasn’t his best year. It was a trying year, a weird year to assess players. Tyler’s a real good kid. I see in the Twitterverse a lot of reports about extracurricular stuff. I made comments to his professionalism and acting more like a professional. You have to remember he’s 21 years old.”

Seguin ran into several speedbumps. The Bruins suspended him for one game in 2011-12 because he missed a team breakfast and meeting in Winnipeg. Following his post-lockout departure from HC Biel in Switzerland, a Swiss newspaper reported Seguin had trashed his apartment.

During a conference call after the trade, Seguin downplayed any criticism of his commitment.

“I come to the rink every day, and I see myself as a professional and work my hardest,” Seguin said. “The first three years of my career has been a big learning curve. I look forward to getting better every day. I’m definitely more motivated coming into a great city like Dallas.”

Seguin played center in junior. The Bruins did not have room for him in the middle. He was not going to displace David Krejci or Bergeron.

Coach Claude Julien tried Seguin at center briefly this past season when Bergeron was out with a concussion. The experiment lasted just over a game. On April 4, Seguin centered Marchand and Jagr, losing nine of 12 faceoffs.

The next game against Montreal, Seguin lasted at center for only a period. He did not display the defensive awareness required of centers in Julien’s system.

The Stars peg Seguin as a center. General manager Jim Nill, who previously worked in Detroit, scouted Seguin in junior. Seguin played in Plymouth, a Detroit suburb. Nill projects Seguin to possibly center Benn on Dallas’s first line.

“We look forward to him taking a leadership role down here in Dallas,” Nill said.

Seguin could find his game in Dallas under ex-Buffalo coach Lindy Ruff. At center, Seguin believes he will have more speed leaving the defensive zone to create scoring chances. At wing, Seguin had to start and stop.

If Seguin hits his rhythm, it won’t surprise the Bruins.

“He’s a tremendous package of skill and speed,” Chiarelli said. “It will go north. His game is more conducive to having ice on both sides.”